Industrial power circuit protection is currently provided by thermal, magnetic, electromechanical and electronic circuit interruption devices tailored to respond to an inverse time-overcurrent function. A long-time delay before circuit interruption is provided for a low overload current value while a shorter time delay is provided for a higher overload current. Upon the occurrence of a short circuit current, the circuit must be interrupted instantaneously.
The thermal and magnetic trip units used within circuit breakers fairly approximate a time-overcurrent relationship which varies with the first power of time and with the second power of current (I.sup.2 t), whereas the magnetic induction discs used within electromechanical relays, for example, follow a polynomial relationship between current and time approximating the I.sup.2 t relation to a lesser extent. A good description of the operation of an electromechanical relay is found within a paper entitled "Digital Inverse Time Overcurrent Relay Using Counters" by M. Ramamoorty, which appeared in the ECI Journal, EL, 1980.
Time-overcurrent trip parameters for electronic circuit interruption devices of the so-called "analog" type use the delay characteristics of an electronic component such as a capacitor to determine the longtime and short-time delays before circuit interruption. U.S. Pat. No. 4,266,259 describes one such analog circuit interrupter.
Time-overcurrent trip parameters for electronic circuit interrupters of the so-called "digital" type are stored in electronic memory and are utilized within a microprocessor to provide long-time and short-time interruption. One example of a digital circuit interrupter is found within U.S. Pat. No. 4,672,501.
When an electronic circuit interrupter is used as a main circuit interrupter within power distribution systems employing magnetic induction discs within branch circuits, some means of coordination is ordinarily required in order for the magnetic induction disc closest to the fault to interrupt the branch circuit before the electronic circuit interrupter responds to interrupt the main circuit.
U.S. patent application Ser. No. (41PR-6558) entitled "Digital Circuit Interrupter with Selectable Trip Parameters" describes logarithmic and linear interpolation algorithms used to tailor a trip-time curve to coordinate with non-electronic circuit interruption devices. This application is incorporated herein for purposes of reference.
Such electronic circuit interrupters having triptime curves tailored to protect industrial power distribution systems, find only limited application in those systems containing large electric motors. For example, a high in-rush current is required during the starting phase of the electric motor, which appears to the electronic circuit interrupter as a short-time fault condition. The electronic circuit interrupter begins timing out a short-time fault and, in some instances, can interrupt the circuit before the motor current subsides to a low, steady state condition.
Prior Art thermal overload relays and magnetic relays as well as dual element fuses can be selected to provide adequate motor protection without interrupting the circuit during the initial high in-rush currents during motor starting.
An early attempt to employ an electronic circuit interrupter having a trip-time characteristic similar to a magnetic induction disc is found within U.S. Pat. No. 4,275,445. Electronic circuit interrupters having both overcurrent protection as well as protective relay function are found in U.S. Pat. No. 4,509,088 which Patent is incorporated herein for purposes of reference.
The present invention improves over electronic circuit interrupters and magnetic relays by providing a time-trip curve specifically tailored for circuits employing electric motors.